Bone conduction is the principle of transmitting vibrations via the skull bone to the inner ear, i.e., the cochlea. For a bone conduction hearing aid the vibrations are transmitted through the skull bone, and since the cochlea is a winding in the bone, the vibrations go directly to the cochlea. This means that for bone conduction the normal transmission chain with the ossicular bones, for example the stapes, is not directly involved in the transmission of the vibrations to the cochlea. Hearing aids that for example connect to the middle ear ossicular bones are not bone conduction hearing aids.
A direct bone conduction hearing aid is a type of bone conduction hearing aid where the vibrator of the hearing aid is firmly connected to the skull bone so that the vibrations from the vibrator do not have to go through the skin to reach the skull bone. A traditional bone conductor where the vibrations have to go through the skin to reach the skull bone is less efficient than a direct bone conductor since the vibrations will be damped when going through the skin.
The most common type of direct bone conductors consist of an external hearing aid with a vibrator that is connected to a skin-penetrating abutment that is connected to a screw-shaped fixture anchored in the skull bone. These patients have often problems with skin infections due to the permanent skin-penetrating abutment.
Direct bone conductors with the vibrator implanted under the skin has been suggested in for example U.S. Pat. No. 3,209,081, US2004032962 and in Hearing by Bone Conduction, Stefan Stenfelt, Chalmers University of Technology, 1999. These solutions have however several drawbacks.
The design suggested by Stenfelt has a vibrator unit positioned in the mastoid cavity and the anchoring fixture is anchored deep in the bottom of the mastoid cavity. The problem with this anchoring is that the bone in this area is very soft, so the anchoring will be quite weak. If the anchoring fixture is placed deep in the mastoid cavity, it will also be difficult to get access to the anchoring fixture if the vibrator unit needs to be removed.
In the solution suggested in US2004032962, the vibrator is placed on the outside of the skull bone and the implantable arrangement is contained in one unit. Since it is very difficult to do a very flat vibrator, the aesthetics of this solution will not be very good. To do the whole implantable arrangement in one unit is also not a good solution since the vibrator, which is usually an electromagnetic vibrator, should not be placed together with the inductive energy transmission coil since it may interfere with the function of the inductive energy transmission.